The present invention relates generally to nuclear reactors and more particularly to grid spacers for use in fuel bundles employed in such reactors for maintaining fuel elements in parallel array therein. These grid spacers are important in the design of nuclear power reactor fuel cores for maintaining the fuel-containing elements in a fixed spaced relationship within the core so that optimum heat generation is achieved over an extended period of time during use of the reactor. If the predetermined special relationship of the elements is not maintained, then satisfactory flow of coolant through the core and about the fissionable fuel material will not be achieved. Consequently, this will result in poor heat transfer efficiency. In connection with maintaining proper flow of coolant in the reactor, there must be a minimization of obstruction to the coolant flow along the fuel elements.
To direct the flow of coolant in the proper direction along the fuel elements, there may be provided mixing vanes which are associated with the grid spacers and are angularly oriented with respect to the direction of coolant flow such that the coolant is caused to assume a desired flow pattern to provide for maximum cooling. Generally, these vanes have been designed to be essentially solid, such that separation of the fluid coolant on the downstream side of the mixing vane results in an overall separation of coolant fluid flow. Pressure differentials resulting from differences in the acceleration of the flow on the upstream and downstream sides of the vanes cause flow separation on its downstream side. This in turn reduces the extent of fluid directional change on the downstream side thereby reducing the extent of interchannel mixing. Because of the fluid separation there will be a corresponding increase in pressure drop of the downstream fluid as well as an obstruction to the directional movement of the fluid between subchannels of the reactor.